STIS slits of the observations compared to the relative positions of the binary components. Only the two smaller slit configurations from Table 2 are shown for both pointings. The position of Ab at different epochs is indicated by filled circles (blue: Sana et al. 2014, red: Mason et al. 2009, green: Nelan et al. 2010). The position of Ab may be extrapolated to the epoch of our observation (2014.8), and is obviously covered by even the smallest STIS slit. The centering procedure with STIS would peak on the center of (visible) light of the binary, rather than placing HD 93129Aaexactly at (0,0). This strengthens the argument that Aa and Ab were both entirely covered by all the STIS apertures.